Crosslinked sulfonated polyimide films

ABSTRACT

The present invention relates to a new sulfonated polyimides, more specifically to new methods for preparing the polyimides, and cation exchange membranes containing the polyimides. The sulfonated polyimides of the presented invention have excellent proton conductivity and low preparation cost. In particular, the sulfonated polyimides can be used as polymer electrolyte membrane in hydrogen or direct methanol fuel cell for electric vehicles and portable power sources operated with electric energy.

FIELD OF THE INVENTION

[0001] The present invention relates to sulfonated polyimides, which canbe applied for the preparation of ion exchange membranes of fuel cells.

BACKGROUND OF THE INVENTION

[0002] Fuel cells using solid polymer electrolytes were proposed in1950s and have been developed ever since for the purpose of supplyingspacecraft with energy.

[0003] Beyond the generation of power for spacecraft, interest in fuelcells has progressed. Particularly, the automobile industry has interestin them for two reasons. The first reason is related to the increasedconcern for avoiding pollution caused by internal combustion engines. Infact, it is very difficult to prevent all discharges caused by internalcombustion engine, such as nitrogen oxides, hydrocarbons caused byincomplete combustion and acidic compounds by means of all theimprovements that one can expect through better control of combustion.The second reason, for the long term, is to research motors that usefuel other than fossil fuel which is known not to last forever.

[0004] Any fuel cell system based on hydrogen or methanol can respond tothe concerns mentioned above. The sources of fuel, hydrogen andmethanol, are potentially inexhaustible and their electrochemicalcombustion only produce water.

[0005] The schematic assembly of a fuel cell that produces electricalenergy and water at the same time is represented in FIG. 1.

[0006] The ion exchange type of membrane formed from a solid polymerelectrolyte (1) is used to separate the anode compartment (4) whereoxidation of the fuel such as hydrogen or methanol occurs according tothe equation:

2H₂→4H⁺+4e ⁻ (hydrogen fuel cell)

CH₆OH+H₂O→CO₂+6H⁺+6e ⁻ (direct methanol fuel cell)

[0007] from the cathode compartment (5) where the oxidant such as oxygenis reduced according to the equation:

O₂+4H⁺+4e ⁻→2H₂O (hydrogen fuel cell)

3/2O₂+6H⁺+6e ⁻→CO₂+2H₂O (direct methanol fuel cell)

[0008] with production of water (7) while the anode and the cathode areconnected through external circuits(6).

[0009] The anode (8) and the cathode (9) are essentially constituted bya porous support, for example made of carbon, on which particles of anoble metal such as platinum or ruthenium are deposited.

[0010] The membrane-electrode assembly (MEA) is a very thin assemblywith a thickness of the order of a millimeter. Each electrode issupplied from the rear with the gases using a fluted plate withserpentine flow path. One very important point is to properly maintainthe membrane in an optimum hydrated state so as to ensure maximum protonconductivity.

[0011] The membrane has a double role. On one hand, it acts as a protonconducting polymer permitting the transfer of hydrated proton (H₃O⁺)from the anode to the cathode. On the other hand, it effectivelyseparates oxygen, hydrogen and/or methanol as a buffer. Therefore, thepolymer constituting the membrane must therefore fulfill a certainnumber of conditions relating to its mechanical, physico-chemical andelectrical properties.

[0012] First, the polymer must be able to be prepared into thinmembranes with a thickness of 50 and 100 micrometers, which are denseand without defects. Its mechanical properties, especially tensilestress, modulus and flexibility must make it compatible with preparationcondition of the MEA which is to be clamped between metal frames.Further, the properties must be conserved simultaneously from a dry to ahydrated state.

[0013] In addition, the polymer must have good thermal stability tohydrolysis and exhibit good resistance to reduction and oxidation up to100° C. In particular, in order to be used for direct methanol fuelcell, the polymer electrolyte membrane must not allow methanol to passthrough the membrane from anode to cathode.

[0014] Finally, the polymer must have high ionic conductivity, which isprovided by acidic groups such as phosphoric acid groups and sulfonicgroups linked to the polymer chain. Therefore, these polymers willgenerally be specified by their equivalent mass, that is to say, acidequivalent per the weight of polymer in grams (Ion Exchange Capacity).

[0015] Since 1950, numerous families of polymers or sulfonatedpolycondensates have been tested as electrolyte membranes for fuel cell.At present the relationships among chemical structure, film morphologyand performance are established.

[0016] At first, sulfonated phenolic type resins prepared by sulfonationof polycondensed products such as phenol-formaldehyde resins were used.

[0017] The membranes prepared with these products are advantageous interms of low cost, but they do not have sufficient stability to hydrogenat 50-60° C. for applications of long duration.

[0018] Next one turned towards sulfonated polystyrene derivatives whichhave greater stability in comparison with those of the sulfonatedphenolic resins, but the sulfonated polystyrene derivatives aredisadvantageous in that they cannot be used at a higher temperature than50-60° C.

[0019] At the present time, the best results are obtained withcopolymers that have linear perfluorinated main chain in back bone andgraft side chains with sulfonic acid groups.

[0020] These copolymers are commercially available under the trademarkNafion from the Du Pont Company or ACIPLEX-S from Asahi ChemicalCompany. Others are experimental products, such as the membrane named“XUS” by the DOW Company.

[0021] Such polymers containing perfluorinated sulfonic acid groups,which have been the subject of numerous developments, conserve theirproperties for several thousands of hours between 80 and 100° C.

[0022] The polymers of the Nafion type can be obtained byco-polymerization of two fluorinated monomers one of which carries thesulfonic acid groups. Other routes for obtaining perfluorinatedmembranes have been explored in documents by G. G. Scherer: Chimia, 48(1994), p. 127-137; and by T. Monose et al., U.S. Pat. No. 4,605,685. Itinvolves the grafting of styrene or fluorinated styrene monomers ontoprevious sulfonated fluorinated polymers. These membranes haveproperties close to those of fluorinated co-polymers.

[0023] However, such Nafion type polymers may be limitedly applied inthe manufacture of direct methanol fuel cell because methanol transferfrom anode to cathode can easily occur even when that the concentrationof methanol is very low, resulting in poor performance.

[0024] In addition, U.S. Pat. No. 6,245,881 shows various sulfonatedpolyimides prepared by copolymerization with diamine monomers havingsulfonic acid groups. The publication reports that these sulfonatedpolyimides have excellent thermal stability and resistance to reduction,as well as high ion exchange capacity up to 2.5 meq/g.

[0025] However, there exist limited kinds of diamine monomers havingsulfonic acid groups. In addition, the solubility and reactivity ofthose monomers are so poor that they cannot be easily resolved in mostsolvents except m-cresol, and the degree of polymerization is too low toform an adequate film.

[0026] The solubility of the monomers may be improved by substitutingthe hydrogen ion of the —SO₃H group by +1 metal ion such as Li+, Na+ andK+. These modified monomers become soluble in other solvents such asdimethylsulfoxide (DMSO). However, the polymers prepared from suchmonomers have poor solubility in most other solvents, and the metalsubstituted sulfonic acid group cannot be easily returned to itsoriginal form, SO₃H in order to be used as a cation exchange membrane.

[0027] Further, because of the strong rigidity of polyimides whosebackbone structures are basically composed of aromatic monomers, theintroduction of SO₃H groups into their main chains prevents themorphologies of the produced films from being uniform.

[0028] From the above, it can be known that the polymers for themanufacture of effective polymer electrolyte membranes must have highproton conductivity, excellent thermal and mechanical properties and lowgas permeability. Also, their chemical structures must be able toprevent the leakage of fuel such as methanol.

SUMMARY OF THE INVENTION

[0029] The purpose of the present invention is to provide polymers,which can solve the above problems and meet the above requirements. Thefurther purpose of the present invention is to provide membranesprepared with these polymers and fuel cells having these membranes.

[0030] In order to achieve the above purposes, the inventors formed acrosslinked sulfonated polyimide (Formula 2) by cross-linking the mainchains of a polyimide composed of the repeat unit (Formula 1) using across-linking agent (B) having a sulfonic acid group.

BRIEF DESCRIPTION OF THE INVENTION

[0031]FIG. 1 represents a schematic membrane electrode assembly of afuel cell producing electrical energy and water at the same time.

[0032] (1) shows a membrane formed from a solid polymer electrolyte,

[0033] (4) shows the anode compartment where oxidation reaction of thefuel occurs,

[0034] (5) shows the cathode compartment where the oxidant is reduced,

[0035] (6) shows an external circuit,

[0036] (7) shows the water produced,

[0037] (8) shows the anode of the fuel cell, and

[0038] (9) shows the cathode of the fuel cell.

DETAILED DESCRIPTION OF THE INVENTION

[0039] The present invention provides a sufonated polyimide comprisingthe repeating units of Formula 2. In Formula 1 and Formula 2, A₁ and A₂can be identical or different. Each formula represents

[0040] i) a tetravalent aromatic radical which includes at least onearomatic carbon ring having 6 to 10 carbon atoms and is substituted byone or more substituents chosen from among alkyl and alkoxy groupshaving 1 to 10 carbon atoms and halogen atoms, or

[0041] ii) a tetravalent aromatic radical which includes at least onearomatic carbon ring having 5 to 10 atoms including one or moreheteroatom(s) chosen from among S, N and O and is substituted by one ormore groups chosen from among alkyl and alkoxy groups having 1 to 10carbon atoms and halogen atoms.

[0042] Examples of groups A₁ and A₂ are represented in the followingstructures (1)-(27).

[0043] The heteroatoms are chosen from the following groups.

[0044] The Ar₁ is a divalent aromatic radical or a mixture of divalentaromatic radicals substituted by —CO— group(s) or —O— group(s).

[0045] Examples of Ar₁ groups are represented in the followingstructures (28)-(34).

[0046] The Ar₂ is i) a divalent aromatic radical which includes at leastone aromatic carbon ring having 6 to 10 carbon atoms and is substitutedby one or more substituents chosen from among alkyl and alkoxy groupshaving 1 to 10 carbon atoms and halogen atoms, or ii) a divalentaromatic radical which includes at least one aromatic carbon ring having5 to 10 atoms including one or more heteroatoms chosen from among S, Nand O and is substituted by one or more substituents chosen from amongalkyl and alkoxy groups having 1 to 10 carbon atoms and halogen atoms.

[0047] Examples of the Ar2 are represented in the following structures(35)-(76).

[0048] The B is a divalent aliphatic radical with a N atom, having asulfonic acid group —SO₃H and two or more groups selected from the groupconsisting of ether group —O— and carbonyl group —CO—. Examples of the Bare as follows. (77-80)

[0049] As a catalyst for the crosslinking reaction,1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide hydrochloride (EDC) canbe used if the crosslinking is formed by ester bonding, while NaH orPPh3 can be used if the crosslinking is formed by ether bonding.

[0050] The X and Y, repeating number of the repeating unit respectively,are a whole number from 2 to 20 and a whole number from 2 to 30, inorder.

[0051] The molecular weights of the polyimides according to the presentinvention range from 10,000 to 100,000 and more preferably from 20,000to 70,000.

[0052] The sulfonated polyimide according to the present invention, witha crosslinked structure of the main chain, has an improved thermalstability and an excellent mechanical property. The sulfonated polyimideof the present invention is resistant to hydrolysis in acidic conditionand has excellent stability in high temperatures such as above 100° C.and good durability.

[0053] In the present invention, strong acid functional groups such assulfonic acid group are introduced into the polyimide by a crosslinkingreaction, which solves the problem of the conventional method to preparesulfonated polyimide in which direct sulfonation in strong acidcondition leads to chain scission of the main chain of the polymer, andthus, film formation is prevented. In addition, in the presentinvention, the main chains of the polyimide can be crosslinked by analiphatic compound with a sulfonic acid group. In this regard, thepolymer may effectively incorporate an ion exchangeable functional groupand be used as a cation exchange membrane.

[0054] The polyimide according to the present invention has a high ionexchange capacity (IEC) above 0.4 meq/g. A polyimide with ion exchangecapacity higher than 1.17 meq/g may be prepared by controlling theamount of the crosslinking agent.

[0055] As the crosslinking reaction result in limiting the distancesamong the main chains and increasing the rigidity of the polymers,relatively larger molecules such as methanol cannot permeate through thepolymers. In addition, the polyimides according to the present inventionhave good thin film-forming characteristics and excellent resistance toreduction and oxidation.

[0056] Because of the above mentioned properties, the polyimidesaccording to the present invention may fully meet the requirements forthe polymer electrolyte membrane, which is an important part in a fuelcell.

[0057] The polymer electrolyte membrane prepared using the presentinvention has a very different structure in comparison with those of theconventional ones. More specifically, the present invention providesvery different type of polymers for cation exchange membrane, inparticular, polymer electrolyte membrane for fuel cell, compared withthe polymers used in the art.

[0058] In particular, the polyimides according to the present inventioncan be prepared by methods which are generally used for an industrialscale at a relatively lower cost. Thus, it can be expected that thepresent invention can lower the cost of the membrane or the MEA.

[0059]FIG. 1 schematically shows a polymer electrolyte fuel cell and itsmain constitutional element—a membrane electrode assembly.

[0060] The polyimide according to the present invention can be preparedby any methods known to a person skilled in the art for the preparationof polyimides in general.

[0061] Examples of known methods for the preparation of polyimides areas follows:

[0062] (1) reaction of a di-anhydride and a di-amine

[0063] (2) reaction of a di-acid diester and a di-amine.

[0064] It is obvious that the polyimides according to the invention canbe prepared by the methods derived from the previously mentioned methodsor by other methods that can be used for the synthesis of polyimides.

[0065] A person skilled in the art can easily carry out a modificationor optimization of the methods known and described in the literature.

[0066] The crosslinked polyimide of the present invention may beobtained by preparing a polyimide in a two-step condensation processusing a di-anhydride and a di-amine and then crosslinking the obtainedpolyimide with a crosslinking agent.

[0067] In an alternative method, a di-amine reacts with a crosslinkingagent having a sulfonic acid group using a catalyst and then condenseswith a di-anhydride. The polyimide of the present invention may beprepared by copolymerization with this condensed product and a polyimideobtained from condensation of a di-amine and a di-anhydride, which arerespectively the same or different type of the above ones.

[0068] These methods, which are currently used in an industrial scale,may be used in order to prepare the polyimides of the present inventionwith just a slight modification.

[0069] The following examples include several methods for preparing thecondensed polyimides of the present invention. However, the scope of theinvention is not limited to these examples.

EXAMPLE 1

[0070] A 250 ml reactor fitted with a Teflon stirring system, an inletfor an inert gas, such as nitrogen, and a sample inlet was prepared tocarry out polyimide condensation reaction and placed in an oil bath toconstantly maintain the reaction temperature.

[0071] The reactor was charged with 0.61 g (4 mmol) of 3,5-diaminobenzoic acid (DBA) and added by N-methyl pyrrolidone (NMP) as a solvent.After complete dissolution, 3.22 g (10 mmol) of 3,3,4,4-benzophenontetracarboxylic dianhydride (BTDA) powder was slowly added into thesolution. After the reaction was maintained for about 1 hour, 1.20 g (6mmol) of oxydianiline (ODA) was further added. After the reaction wasmaintained for 3 hours, a deep brown viscous solution was obtained. Thissolution was slowly added by a solution of 0.43 g (2 mmol) of N,N-bis(2-hydroxyethyl)-2-aminoethane sulfonic acid (BES) powder in NMP andmaintained for 1 hour at 60-90° C.

[0072] The solution was casted on a glass plate and oven-cured for 2hours at 110° C., for 1 hour at 150° C., for 1 hour at 200° C. and for 1hour at 250° C., in sequence. Then, vacuum drying was carried out in avacuum oven at 60° C. for 24 hours in order to completely remove theresidual solvent. A transparent sulfonated polyimide membrane with anion exchange capacity (IEC) of 1.19 meq/g was obtained.

EXAMPLE 2

[0073] The method of Example 1 was used to prepare a sulfonatedpolyimide by reacting 0.76 g (5 mmol) of DBA, 3.22 g of BTDA (10 mmol),1 g of ODA (5 mmol) and 0.53 g of BES (2.5 mmol). The IEC of theobtained membrane was 1.25 meq/g.

EXAMPLE 3

[0074] The method of Example 1 was used to prepare a sulfonatedpolyimide by reacting 0.91 g (6 mmol) of DBA, 3.22 g of BTDA (10 mmol),0.8 g of ODA (4 mmol) and 0.64 g of BES (3 mmol). The IEC of theobtained membrane was 1.33 meq/g.

EXAMPLE 4

[0075] The method of Example 1 was used to prepare a sulfonatedpolyimide by reacting 1.07 g (7 mmol) of DBA, 3.22 g of BTDA 10 mmol),0.6 g of ODA (3 mmol) and 0.75 g of BES (3.5 mmol). The IEC of theobtained membrane was 1.41 meq/g.

EXAMPLE 5

[0076] The method of Example 1 was used to prepare a sulfonatedpolyimide by reacting 1.22 g (8 mmol) of DBA, 3.22 g of BTDA (10 mmol),0.4 g of ODA (2 mmol) and 0.85 g of BES (4 mmol). The IEC of theobtained membrane was 1.48 meq/g.

EXAMPLES 6-10

[0077] In these Examples, the proton conductivities of the membranes ofpolyimides having sulfonic acid groups were obtained by measuring theirA.C. impedances at 30° C., 45° C., 60° C., 75° C. and 90° C. usinggalvanostatic four-point probe electrochemical impedance spectroscopytechnique and calculating their proton conductivities using thefollowing equation. Table 1 shows the proton conductivities calculatedfrom the measured impedances of the sulfonated polyimide membranesprepared from Examples 1 to 5, $R = {\rho \frac{l}{S}}$

[0078] {R=resistance (Ω), ρ: specific resistance, l: the distancebetween the electrodes (cm), S: effective surface area(cm²)}$\frac{1}{\rho} = {\sigma = \frac{l}{R\quad S}}$

[0079] {σ: conductivity (1/Ω cm=S/cm)} TABLE 1 * proton conductivity(10⁻³ S/cm) 30° C. 45° C. 60° C. 75° C. 90° C. Example 6 4.95 5.76 8.029.39 11.30 Example 7 9.68 11.80 14.65 18.23 22.49 Example 8 15.74 19.8322.77 27.92 32.44 Example 9 22.32 25.37 26.84 29.56 35.67 Example 1028.10 33.97 39.38 47.43 78.82

EXAMPLES 11-15

[0080] The Examples relate to the application of the polyimide membranehaving a sulfonic acid group according to the present invention as aproton exchange membrane of a direct methanol fuel cell.

[0081] In order to evaluate the methanol crossover, each of thesulfonated polyimide membranes prepared from the Examples 1-5 was cut to4 cm in diameter and then adhered to a silicon rubber ring 4 cm inexternal diameter and 2.5 cm in internal diameter using epoxy adhesive,which was then positioned in a two chamber diffusion cell and closelysealed.

[0082] One of the two chambers was filled with distilled water and theother was filled with 10 M methanol aqueous solution. The concentrationof each chamber was constantly maintained using a magnetic stirring bar.The sample of the chamber filled with distilled water was collected atregular time intervals using a 1 μl micro injector and analyzed in a gaschromatography equipped with a thermal conductivity detector.

[0083] In addition, in order to evaluate the methanol permeabilitydepending on temperature, methanol crossovers of the samples weremeasured in a thermostat at a regulated temperature between 30-100° C.

[0084] Table 2 shows the methanol crossovers of the samples of Examples1 to 5 (Examples 11-15). The values of Table 2 are % concentrations ofmethanol measured by GC after 1 week. TABLE 2 * methanol concentration(%) measured by GC. 30° C. 45° C. 60° C. 75° C. Example 11 X X X XExample 12 X X X X Example 13 X X X X Example 14 X X X X Example 15 X XX X

EXAMPLES 16-20

[0085] These Examples relate with the application of the sulfonatedpolyimide membranes according to the present invention as an ionexchange membrane of a direct methanol fuel cell.

[0086] The hydrolytic stability at high temperature conditions of thesulfonated polyimide membranes prepared from Examples 1-5, was evaluatedby measuring the changes in outward appearance, weight, physicalproperties and proton conductivity after dipping the membranes in boiledwater at 100° C. for 8 hrs. As a result, no significant changes wereobserved.

EXAMPLES 21-25

[0087] The Examples relate with the application of the sulfonatedpolyimide membranes according to the present invention as an ionexchange membrane of a direct methanol fuel cell.

[0088] The stability for peroxide radicals generated when a directmethanol fuel cell is operated, was evaluated for the sulfonatedpolyimide membranes prepared from Examples 1-5. The changes in outwardappearance, weight, physical properties and proton conductivity of themembranes were measured after dipping them in a solution of 3 wt % ofhydrogen peroxide and 0.1 wt % of ferrous ammonium sulfate at 70° C. for8 hrs. As a result, no significant changes were observed.

COMPARATIVE EXAMPLE 1

[0089] The membrane made of Nafion 115 with the following structure (81)was used in order to compare its proton conductivities with those of themembranes according to the present invention.

[0090] The ion conductivities of the membrane under the same operationconditions as those for Example 6, were 20.1, 24.3, 30.2, 41.4 and53.03(10⁻³ S/cm) at 30° C., 45° C,, 60° C., 75° C. and 90° C.

[0091] When the membrane made of Nafion 115 was tested under the samecondition of Example 11 for evaluating methanol crossover, the methanolconcentration measured at 30° C. after 4 hrs was 40%.

COMPARATIVE EXAMPLE 2

[0092] The membrane prepared from sulfonated polysulfone with thefollowing structure was prepared in order to compare its hydrolyticstability at high temperature conditions and peroxide radical stabilitywith those of the membranes according to the present invention.

[0093] The hydrolytic stability at high temperature conditions wastested under the same conditions of operation as those for Examples16-20. As a result, the membrane prepared from the sulfonatedpolysulfone was readily cracked.

[0094] In addition, the peroxide radical stability was tested under thesame conditions of operation as those for Examples 21-25. Likewise, themembrane prepared from the sulfonated polysulfone was readily cracked.

COMPARATIVE EXAMPLE 3

[0095] The membrane prepared from sulfonated polyether ether ketone withthe following structure was prepared in order to compare its hydrolyticstability at high temperature conditions and peroxide radical stabilitywith those of the membranes according to the present invention.

[0096] The hydrolytic stability at high temperature conditions wastested under the same conditions of operation as those for Examples16-20. As a result, the membrane was readily cracked.

[0097] The peroxide radical stability was tested under the sameconditions of operation as those for Examples 21-25. Likewise, themembrane was readily cracked.

COMPARATIVE EXAMPLE 4

[0098] The membrane prepared from sulfonated polyether imide with thefollowing structure was prepared in order to compare its hydrolyticstability at high temperature conditions and peroxide radical stabilitywith those of the membranes according to the present invention.

[0099] The hydrolytic stability at high temperature was tested under thesame conditions of operation as those for Examples 16-20. As a result,the membrane was readily cracked.

[0100] The peroxide radical stability was tested under the sameconditions of operation as those for Examples 21-25. Likewise, themembrane was readily cracked.

[0101] As described above, the sulfonated polyimides according to thepresent invention in which the main chains of the polyimides arecrosslinked and the sulfonic acid groups are effectively introduced, canbe applied to polymer electrolyte membranes for direct methanol fuelcell, because they have excellent proton conductivities like Nafion typeperfluorinated polymers which are known to exhibit the highestperformances in the art, without the concern of methanol crossover.

[0102] In addition, since the present invention uses inexpensivemonomers and the introduction of sulfonic acid group is easy, it isexpected that the present invention contributes to mass production ofthe membranes in an industrial scale.

What is claimed is:
 1. A sulfonated polyimide comprising the repeatingunits of Formula 2,

wherein A₁ and A₂ is identical or different, and each represents i) atetravalent aromatic radical which includes at least one aromatic carbonring having 6 to 10 carbon atoms and is substituted by one or moresubstituents chosen from among alkyl groups and alkoxy groups having 1to 10 carbon atoms and halogen atoms, or ii) a tetravalent aromaticradical which includes at least one aromatic carbon ring having 5 to 10atoms including one or more heteroatoms chosen from among S, N and O andis substituted by one or more substituents chosen from among alkylgroups and alkoxy groups having 1 to 10 carbon atoms and halogen atoms;Ar₁ is a mixture of divalent aromatic radicals substituted by —CO— groupor —O— group; Ar₂ represents i) a divalent aromatic radical whichincludes at least one aromatic carbon ring having 6 to 10 carbon atomsand is substituted by one or more substituents chosen from among alkylgroups and alkoxy groups having 1 to 10 carbon atoms and halogen atoms,or ii) a divalent aromatic radical which includes at least one aromaticcarbon ring having 5 to 10 atoms including one or more heteroatomschosen from among S, N and O and is substituted by one or moresubstituents chosen from alkyl groups and alkoxy groups having 1 to 10carbon atoms and halogen atoms; B represents a divalent aliphaticradical with a N atom having a —SO₃H group and two or more groups chosenfrom among —O— group and —CO— group; and the repeating number Xrepresents a whole number from 2 to 20 and the repeating number Yrepresents a whole number from 2 to
 30. 2. The sulfonated polyimideaccording to claim 1, characterized in that its molecular weight is from10,000 to 100,000.
 3. The sulfonated polyimide according to claim 1,characterized in that A₁ and A₂ is identical or different, and eachrepresents i) a benzene ring substituted by one or more substituentschosen from among alkyl groups and alkoxy groups having from 1 to 10carbon atoms and halogen atoms, or ii) two or more benzene ringssubstituted by one or more substituents chosen from among alkyl andalkoxy groups having from 1 to 10 carbon atoms and halogen atoms, andwherein the benzene rings are linked to one another by two or moresingle bonds or divalent groups. Ar₁ is a mixture of divalent aromaticradicals which includes —CO— group or —O— group; Ar₂ represents i) adivalent aromatic group which includes at least one aromatic carbon ringhaving 6 to 10 carbon atoms and is substituted by one or moresubstituents chosen from among alkyl groups and alkoxy groups having 1to 10 carbon atoms and halogen atoms, or ii) a divalent aromatic groupwhich includes at least one aromatic carbon ring having 5 to 10 atomsand one or more heteroatoms chosen from among S, N and O, and issubstituted by one or more substituents chosen from among alkyl groupsand alkoxy groups having 1 to 10 carbon atoms and halogen atoms; and Brepresents a divalent aliphatic radical with a N atom, having a —SO₃Hgroup and two or more groups chosen from among —O— group and —CO— group.4. The sulfonated polyimide according to claim 1, characterized in thatit is crosslinked by the two or more groups chosen from among hydroxylgroup and carbonyl group of B.
 5. The sulfonated polyimide according toclaim 3, characterized in that the divalent of Ar₁ and Ar₂ is chosenfrom among; i) a divalent group derived from a linear or branched alkylgroup with 1-10 carbon atoms, optionally substituted by hydroxyl groupsor halogen atoms chosen from among F, Cl, Br and I ii) a divalent groupin which the heteroatom is chosen from among O and S.
 6. The sulfonatedpolyimide according to claim 3, characterized in that Ar₁ is a benzenering with —CO— group and Ar₂ is a diphenyl ether group.
 7. Thesulfonated polyimide according to claim 1, characterized in that Ar₁ isa benzene ring with —CO— group and the Ar₂ is a benzene ring.
 8. Thesulfonated polyimide according to claim 1, characterized in that Ar₁ isa benzene ring with —CO— group and Ar₂ is a diphenyl methane group. 9.The sulfonated polyimide according to claim 1, characterized in that Ar₁is a benzene ring with —CO— group and Ar₂ is diphenyl disulfonic acidgroup.
 10. The sulfonated polyimide according to claim 1, characterizedin that A₁ and A₂ are two benzene rings linked to each other by acarbonyl group.
 11. The sulfonated polyimide according to claim 1,characterized in that A₁ and A₂ are benzene rings linked to each otherby one or more divalent perfluoroalkyl or perfluoroalkylene groups. 12.The sulfonated polyimide according to claim 1, characterized in that A₁and A₂ are both benzene rings.
 13. The sulfonated polyimide according toclaim 1, characterized in that A₁ and A₂ are both naphthalene rings. 14.The sulfonated polyimide according to claim 1, characterized in that A₁and A₂ are both benzene rings linked to each other by a sulfonyl group.15. The sulfonated polyimide according to claim 1, characterized in thatA₁ is a benzene ring and the A₂ is two benzene rings linked to eachanother by a carbonyl group.
 16. The sulfonated polyimide according toclaim 1, characterized in that A₁ is a benzene ring and the A₂ is two ormore benzene rings linked to one another by two or more divalentperfluoroalkyl or perfluoroalkylene groups.
 17. The sulfonated polyimideaccording to claim 1, characterized in that A₁ is two benzene ringslinked to each other by a carbonyl group and A₂ is two or more benzenerings linked to one another by one or more divalent perfluoroalkyl orperfluoroalkylene groups.
 18. The sulfonated polyimide according toclaim 1, characterized in that Ar₁ is a benzene ring having —CO— groupand Ar₂ is a diphenylthionyl group.